1. Field of the Invention
The invention relates to a plastification and injection unit in injection-molding machines, for processing of polymer materials, for example rubber. The unit has a plastification screw that is disposed so as to rotate in a screw cylinder. The screw cylinder simultaneously represents the injection piston, which is disposed in an injection cylinder, in an axially displaceable manner. The injection cylinder can be connected with the sprue channel of an injection-molding die by way of a nozzle, and the injection piston has a back-flow barrier.
2. The Prior Art
These types of injection units work according to the so-called FIFO principle (first in-first out). During the plastification phase (filling of the injection cylinder with injection mass that is capable of flow, by means of rotation of the plastification screw), a connection channel from the screw chamber to the injection cylinder is required. To fill the mold, a high pressure must be built up in the injection cylinder, particularly in the case of viscous elastomer masses. In order to prevent return flow of the injection mass in the direction of the screw, the connection channel must be equipped with a kick-back valve (back-flow barrier).
With the known devices, a freely movable part (ball, cone, or the like), which is disposed in a separate chamber between the screw tip and piston tip in the injection piston, is used as a back-flow barrier, according to the model of a traditional hydraulic kick-back valve. This part is pressed against the seal seat by the mass pressure, thereby closing off the connection channel to the screw chamber. During the plastification process, this movable part is moved in the opposite direction by means of the mass pressure, thereby opening the connection channel toward the piston opening.
As stated, the movable part is situated in a separate chamber in the injection piston, thereby resulting in a relatively complicated structure. The piston consists of many individual parts having sensitive fits, sealing surfaces, and precision threads.
The injection channel from the screw cylinder to the piston outlet and the chamber for the movable part can, of course, become clogged during operation, by material that hardens, so that the injection unit frequently has to be cleaned. If this is to happen in a reasonable manner, disassembly for cleaning is unavoidable in the case of the conventional constructions. In this connection, these many individual parts have to be handled carefully. The slightest assembly errors (e.g. overlooked dirt particles or deficient lubrication) very quickly lead to failure of the entire construction. Only by means of the most precise cleaning of the individual parts and application of special grease to the thread flanks can an acceptable useful lifetime be achieved. The regular cleaning of the injection unit that is actually required is therefore very time-consuming and risky at present. A possible production failure is a threat. For this reason, injection units are thoroughly cleaned much too rarely nowadays. The old deposits that result from this, which burst off out of the injection unit and enter the molded parts, cause high scrap rates.